Mixtures of organic liquid components are frequently separated at present on a large scale by multistage distillation (rectification), which is characterized by: high energy demands, relatively large capital plant investment, a variety of maintenance problems, and severe environmental problems that contribute to the energy demands. For example, high temperature multistage distillation increases thermal loads on the cooling system of a chemical plant and generally contributes to water and air pollution, while regulations, which are designed to mitigate these problems, often constitute an additional energy burden on the separation process.
The need to use energy efficiently, and to conserve resources, have focused attention on by-products of the chemical and petroleum industries. Most of these by-products consist of mixtures of liquid organic compounds, whose separation is often complicated and costly. Some of the by products have been considered, until recently, as expendable and disposable, or were burned to provide an expensive energy source. The development of a low-cost technique of separating such mixtures would clearly be of great benefit.
Separation processes which involve the use of porous and/or semi-permeable membranes for separating compounds from each other have been used increasingly in recent years. Whereas conventional techniques such as distillation, adsorption, liquid-liquid extraction and crystallization are often relatively inadequate and uneconomical, application of membrane technology can save in process costs because energy consumption is low, and raw materials can be recovered and reused. Moreover, the process can be carried out continuously, and disposal problems can be reduced or eliminated.
Membrane permeation by the pervaporation process involves selective sorption of a liquid mixture into a membrane, diffusion through the membrane, and desorption into a vapor phase on the permeate side of the membrane. Because of the interesting potential applications of pervaporation techniques, for example, to the separation of organic liquid mixtures, attempts have been made to discover commercially acceptable membranes, and numerous studies dealing with separating organic liquids using pervaporation processes have been reported.
U.S. Pat. No. 4,073,754 (Cabasso et al.) describes the use of pervaporation membranes for separation of aromatic hydrocarbons such as benzene, cyclohexane, ethanol, from other organic solvents, e.g., aliphatic hydrocarbons such as cyclohexane, decalin, heptane. The membrane materials employed consisted of a polymer alloy of poly (vinylidenechloride-benzyldiethyl phosphone) copolymer and acetyl cellulose; a phosphonylated poly(phenylene) oxide derivative and acetyl cellulose; and poly(2-methyl-6-methylenedimethyl phosphonate-3-bromo-1, 4-phenylene) oxide.
U.S. Pat. No. 4,728,429 (Cabasso, et al.) describes a membrane permeation process for dehydration of organic liquid mixtures using sulfonated ion-exchange polyalkylene membranes, and in particular a pervaporation process where the membrane is prepared by free radical reaction of chlorine and sulfur dioxide with a linear polyalkylene and subsequently either hydrolyzing the produced chlorsulfonated polyalkylene giving a cation-exchange sulfonated polyalkylene, or treating the chlorsulfonated polyalkylene with an amine followed by quaternization, to give an anion-exchange sulfonated polyalkylene, the membrane being between about 25% and about 75% amorphous in structure and having a charge density between about 0.2 meq/g and about 4.5 meq/g.
In U.S. Pat. No. 4,798,674 (Pasternak, et al.), there is described a method for concentrating an aqueous charge solution containing (i) an alcohol having 1 or 2 carbon atoms and (ii) an oxygenate selected from organic ethers, aldehydes, ketones, and esters, by contact with the high pressure side of a non-porous membrane separating layer, across which a pressure drop is maintained, and which is selected from (i) polyvinyl alcohol which has been crosslinked with an aliphatic polyaldehyde containing at least three carbon atoms including those in said aldehyde groups; and (ii) high molecular weight ion exchange resin in membrane form having carbon atoms in the backbone bearing a pendant acid group, which membrane has been contacted with a quaternary ammonium salt containing four hydrocarbyl groups; whereby a lean mixture containing relatively more alcohol and less oxygenate is recovered as permeate from the low pressure side of the non-porous separating layer, and a rich mixture containing relatively less alcohol and more oxygenate is recovered as retentate from the high pressure side of the non-porous separating layer.
In U.S. Pat. No. 4,997,567 (Messalem et al.), there is described a permselective, dimensionally stable, ion-exchange membrane, which selectively separates ions of opposite electric charges passing therethrough, the membrane being an activated polymeric film matrix incorporating fixed anionic and/or cationic groups, in which augmented dimensional stability is provided by an integral network of an inert unactivated polymeric film.
U.S. Pat. No. 5,066,403 (Dutta et al.) relates to a composite membrane containing ion-exchange groups, which may be used for separating azeotropic mixtures and close-boiling liquid mixtures by pervaporation, and which is made by casting perfluorosulfonic acid polymer (i.e. containing a fluorocarbon backbone and sulfonic groups) on a porous matrix of polytetrafluoroethylene (PTFE). JP 58089907 and JP 58089903 describe, respectively, an ion-exchange pervaporation membrane made from fluorinated olefin/fluorovinylpolyether/fluorovinylmonoether terpolymer, and such a membrane where ion exchange capacity changes in the thickness, made from fluorovinyl monomer and fluorovinyl carboxylic monomer.
U.S. Pat. No. 5,643,968 (Andreola et al.) relates to ion-exchange membranes used for a variety of processes including pervaporation, and which are made from a soluble graft copolymer containing a backbone of a first polymer having a main chain containing aromatic rings, and a polymerizable vinyl or ring-containing compound, displaying or convertible to ion-exchange functionality, grafted onto the first polymer.
U.S. Pat. No. 5,755,967 (Meagher et al.) describes the selective removal of acetone and/or butanol from aqueous solutions and mixtures thereof, by a pervaporation process using a membrane comprising silicalite particles embedded in a polymer matrix.
Composite ion-exchange pervaporation membranes are described in JP 61161109 and JP 4104824.
Numerous pervaporation membranes have been described for separating organic liquid components which are not ion-exchange membranes. By way of example, in U.S. Pat. No. 3,930,990 (Brun et al.), components of hydrocarbon mixtures are separated, e.g. by pervaporation through membranes, e.g. from butadiene-acrylonitrile copolymer, comprising groups which complex with one hydrocarbon component. In U.S. Pat. No. 4,802,987 (Black), aromatic hydrocarbons are separated from a mixture with non-aromatic compounds by selective permeation through a polyethylene glycol impregnated regenerated cellulose or cellulose acetate membrane. In U.S. Pat. No. 5,559,254 (Krug et al.), methanol and tetrahydrofuran are separated by pervaporation using, as an organophilic/hydrophilic membrane, a plasma polymerization membrane.
It will be evident to the skilled person that recent developments in the pervaporation art concern, on the one hand, ion exchange membranes from fluorinated polymers, more complex polymers such as graft polymers, or such membranes which are composites (e.g., impregnated or laminated), or, on the other hand, membranes for pervaporation which are not ion exchange membranes at all.
Persons skilled in the art will also be aware that there exist multipurpopervaporation membranes as well as such membranes which are applied for a single purpose, e.g. to separate particular organic mixtures, or to dehydrate mixtures of organic liquids with water. Where a pervaporation membrane has been disclosed solely for such dehydration, it would be unlikely that it could also be applied to the separation of components of organic liquid mixtures, for the following reason. Water is believed to diffuse efficiently through an ion exchange membrane via ion hydration shells, whereas the mechanism of diffusion of an organic liquid must be different from water, because such liquid cannot replace water in the ion hydration shells, and it would rather be expected to diffuse through the polymer phase.
The present inventors have surprisingly found that a modification of an ion exchamge pervaporation membrane, such as is disclosed in U.S. Pat. No. 4,728,429 exclusively for the purpose of dehydration of aqueous organic liquid mixtures, can be utilized effectively for separation of organic components of organic liquid mixtures from each other.
The entire contents of the above-mentioned patents or published patent applications are incorporated by reference herein.
It is an object of this invention to provide a simple and effective process for selectively separating organic liquid components from their mixtures.
Another object of the present invention is to provide a membrane pervaporation technique employing permselective polymer membranes for efficiently separating organic liquid components from their mixtures.
Still another object of the present invention is to provide membrane pervaporation techniques which can be used to achieve almost complete separation of organic liquid components from their mixtures in one stage.
Additional objects and advantages of the invention will become apparent from the description which follows.